Silicon nitride (SiC) refractory materials are important in industry for the excellent heat resistance and refractoriness and are being used in a large amount in, for example, setters for firing tiles, pottery, honeycomb structures, etc.; kiln furniture for firing other than the setters; and saggers.
In firing a to-be-fired material to obtain a ceramic product (e.g. tiles, pottery or honeycomb structures), plurality of block-shaped pillars are generally placed on the corners of the respective setter floors to form a space that is necessary for uniform firing of to-be-fired material, setters and the pillars are alternately piled up to form a multi-layered setter, be-fired material is placed on each setter of the multi-layered setter, and firing is carried out.
Assembly of such a multi-layered setter has often been made manually. However, in recent years, automation and labor saving have been adopted in firing and production lines for ceramic product, and automation has been progressed in transfer of setter and produced articles.
As the base material for the setters and pillars constituting a multi-layered setter, there have been used mainly mullite, alumina and zirconia which can be used at high temperatures (e.g. 1,200 to 1,600° C.). They, however, are insufficient in strengths (bending strength and Young's modulus); therefore, oxide-bonded SiC refractory and nitride-bonded SiC refractory all having higher strengths are in use depending upon the application purposes.
In a multi-layered setter made of such an oxide-bonded SiC refractory or a nitride-bonded SiC refractory, however, it is necessary to make the thicknesses of the setter floors and pillars thick in order to obtain a strength capable of withstanding the external stress which appears at the time of assembling, use and disassembling of the multi-layered setter. This has resulted in a larger weight and lower operating efficiency, a larger heat capacity which makes it difficult to well respond to a recent years' requirement for energy saving, and a lower thermal conductivity which makes difficult the uniform heating of to-be-fired material.
In order to secure the oxidation resistance and strength required for the above-mentioned refractory, a bulk density of 2.6 or more is necessary, and oxides such as alumina and iron oxide have ordinarily been added in an amount of 3 to 10% by mass. When alumina and an iron oxide are added in an amount of 3 to 10% by mass, the amount of glass phase at the bonding portion between SiC and added oxide increases, which contributes to a reduction in creep resistance otherwise required for a refractory material and resusts in a shorter useful life. Further, alumina and an iron oxide have a dispersion effect in compounding raw materials, and also act as a sintering aid; therefore, in the conventional method, when the amount of alumina and an iron oxide added was reduced to 3% or less, there was a problem in the reduction of the bulk density and insufficient strength.
For solving the above problems, there is in use a multi-layered setter made of a (porous or dense) Si-containing material containing, as a metallic Si phase, Si high in strength and superior in heat resistance, oxidation resistance and thermal conductivity; for example, a metallic silicon-silicon carbide composite material (a Si-impregnated SiC refractory) containing Si and SiC as main phases.
In the case of a multi-layered setter made of such a Si-impregnated SiC refractory, the setter and pillars can be thinner and lighter; therefore, a higher operating efficiency and incresed energy saving are expected, and it is possible to obtain a longer useful life and better recycling of setter.
In the case of the multi-layered setter made of a Si-impregnated SiC, however, the use at high temperatures, for example, at 1,400 to 1,600° C. is difficult because the upper limit of temperature for practical use of the base material, the Si-impregnated SiC is less than 1,400° C.
The present invention has been made in view of the above-mentioned problems of prior art and aims at providing a silicon nitride-bonded SiC refractory which has heat resistance, thermal shock resistance and oxidation resistance and which is high in strength and superior in creep resistance and thermal conductivity, and a method for producing such a silicon nitride-bonded SiC refractory.